Hospitals are spending thousands every year on an expensive blood test for Down's syndrome which is no more accurate than a simple ultrasound scan, says research.

Serum screening, the so-called "triple test", has been routinely offered to pregnant women since the early 1990s.

It detects chemicals in the blood which could be a sign of Down's syndrome.

But doctors from a maternity hospital in Southampton have looked closely at how often the test gets the right result.

And their results, published in the British Medical Journal, suggest that the "triple test" is unnecessary.

They found that the normal mid-pregnancy ultrasound scan, which can produce pictures of the baby in the womb, can also pick out in many cases the developing differences between Down's syndrome babies and healthy ones.

Older mums at risk

Older mothers-to-be are more at risk of having a Down's baby, and when the age of the mother was taken into account, an accurate prediction could be made in many more cases than expected.

Dr David Howe, a consultant in foetal/maternal health at the Princess Anne Hospital in Southampton, said that his unit had now stopped offering the triple test.

If a woman wanted one, he said, she would have to pay for it herself.

Each test is thought to cost the NHS between £15 and £20, and many thousands are carried out each year.

Dr Howe said: "We found the main cost was not so much the money, but the amount of time we were spending explaining the test and its consequences to women.

"It is an extremely complicated test, and we were spending easily 10 or 15 minutes going through it."

The "triple test" became popular after a report in the British Medical Journal in 1992 said that the NHS should ensure that the test was available in every hospital in Britain.

Dr Howe said that the test had never been clinically proven to be better than the alternatives.

He said: "The main reason for writing this is that there are a whole series of tests about to come out that are having no proper controlled studies as to their effectiveness."